Inputs

Any SQL WHERE clause, new or
old can appear instead of an
instance variable whenever an instance variable is
permissible in SQL.

action

Any SQL statement, new or
old can appear instead of an
instance variable whenever an instance variable is
permissible in SQL.

Outputs

CREATE

Message returned if the rule is successfully
created.

Description

The Postgresrule system allows one to define an alternate
action to be performed on inserts, updates, or deletions from
database tables or classes. Currently, rules are used to
implement table views.

The semantics of a rule is that at the time an individual
instance is accessed, inserted, updated, or deleted, there is a
old instance (for selects, updates and deletes) and a new
instance (for inserts and updates). If the event specified in the ON clause and the
condition specified in the WHERE
clause are true for the old instance, the action part of the rule is executed. First,
however, values from fields in the old instance and/or the new
instance are substituted for old.attribute-name
and new.attribute-name.

The action part of the rule
executes with the same command and transaction identifier as the
user command that caused activation.

Notes

A caution about SQL rules is in order. If the same class
name or instance variable appears in the event, condition and action parts of a rule, they are all
considered different tuple variables. More accurately,
new and old
are the only tuple variables that are shared between these
clauses. For example, the following two rules have the same
semantics:

ON UPDATE TO emp.salary WHERE emp.name = "Joe"
DO
UPDATE emp SET ... WHERE ...

ON UPDATE TO emp-1.salary WHERE emp-2.name = "Joe"
DO
UPDATE emp-3 SET ... WHERE ...

Each rule can have the optional tag INSTEAD. Without this
tag, action will be performed in
addition to the user command when the
event in the condition
part of the rule occurs. Alternately, the action part will be done instead of the
user command. In this later case, the action can be the keyword NOTHING.

It is very important to note to avoid circular rules. For
example, though each of the following two rule definitions are
accepted by Postgres, the
select command will cause Postgres to report an error because the
query cycled too many times:

Example 19-1. Example of a circular rewrite
rule combination.

CREATE RULE bad_rule_combination_1 AS
ON SELECT TO emp
DO INSTEAD
SELECT TO toyemp;

CREATE RULE bad_rule_combination_2 AS
ON SELECT TO toyemp
DO INSTEAD
SELECT TO emp;

This attempt to select from EMP will cause Postgres to issue an error because the
queries cycled too many times.

SELECT * FROM emp;

You must have rule definition access to a class in order to
define a rule on it. Use GRANT and
REVOKE to change permissions.

The object in a SQL rule cannot
be an array reference and cannot have parameters.

Aside from the "oid" field, system attributes cannot be
referenced anywhere in a rule. Among other things, this means
that functions of instances (e.g., foo(emp) where emp is a
class) cannot be called anywhere in a rule.

The rule system stores the rule text and query plans as text
attributes. This implies that creation of rules may fail if the
rule plus its various internal representations exceed some
value that is on the order of one page (8KB).

Usage

At the time Joe receives a salary adjustment, the event will
become true and Joe's old instance and proposed new instance are
available to the execution routines. Hence, his new salary is
substituted into the action part of the rule which is subsequently
executed. This propagates Joe's salary on to Sam.

Make Bill get Joe's salary when it is accessed:

CREATE RULE example_2 AS
ON SELECT TO EMP.salary
WHERE old.name = "Bill"
DO INSTEAD
SELECT emp.salary
FROM emp
WHERE emp.name = "Joe";

Deny Joe access to the salary of employees in the shoe
department (current_user returns the
name of the current user):

CREATE RULE example_3 AS
ON
SELECT TO emp.salary
WHERE old.dept = "shoe" AND current_user = "Joe"
DO INSTEAD NOTHING;